The invention relates generally to analysis of the properties of liquid hydrocarbons, and more particularly to refinery process control and test apparatus for determining the temperature at which wax crystals first appear in a liquid hydrocarbon sample.
In the refining of certain liquid hydrocarbons such as diesel and fuel oil, wax molecules are found to be present in varying concentrations and forms at different stages of the refining process. Wax can be removed to a large extent by cooling the product to the point where the wax crystallizes and precipitates. The point at which wax crystals begin to form is referred to in the art as the cloud point, characterized by a rapid change in the optical characteristics of the liquid. In order to control the amount of wax remaining in a liquid hydrocarbon product to meet industry specifications for a given type of fuel oil for example, it is important to know the precise cloud point of the liquid hydrocarbon being produced. However, cloud points vary widely with the type of liquid hydrocarbon and are incapable of precise prediction without actually sampling the feedstock and testing its cloud point. Cloud point temperature is used in process control. The amount of wax remaining in the product may affect its quality and indirectly the price it can command in the marketplace. Inadvertently precipitating out far more wax than is required by the specifications for a given fuel oil, for example, can result in selling a very high quality fuel oil at the wrong price. Cooling large quantities of refinery product to remove wax is a costly undertaking requiring huge refrigeration units and tankage requirements. By increasing the accuracy and frequency of cloud point measurement, the refinery can control the product more closely with respect to sales specifications, thus optimizing quality control and maximizing the refinery's more profitable fractions.
In the past, cloud points have been analyzed in refineries by laboratory technicians. In many cases, samples are taken only once per shift. Meanwhile process control variables can range far from the optimum level. One of the major problems facing refinery companies is the measurement of cloud point of straight run products produced from a number of different crudes with differing characteristics. With frequently varying feedstock properties, the cloud point may change abruptly. Moreover, because of the varying optical properties of the feedstock, the threshold for optical detection of cloud point may also vary abruptly.